Blow It Out Your Sides: Caterpillars Can Whistle

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Caterpillars apparently can whistle, letting out squeaks that can
fend off attacking birds, scientists have now found.

They don't whistle by puckering their lips and blowing, since
they don't have lips. Instead, they blow out their sides,
researchers said.

Scientists have known for more than 100 years that many
caterpillars can generate clicking or squeaking noises. However,
researchers have only recently begun to experimentally
investigate how these noises are made and what roles they might
play.

Neuroethologist Jayne Yack at Carleton University in Ottawa had
shown that silk-moth caterpillars ( Antheraea
polyphemus ) make clicking sounds by
snapping their mandibles together. Now she and her colleagues
for the first time have revealed that walnut sphinx caterpillars
(Amorpha juglandis) can toot from their sides.

Using high-speed video, the researchers noticed they pulled their
heads back to compress the body cavity while they whistled.
Unlike reptiles, birds and mammals, insects don't breathe using
their mouths, but with holes in their sides known as spiracles,
and the scientists reasoned they were forcing
air out these holes to whistle, generating squeaking noises.

To confirm their idea, researcher Veronica Bura at Carleton
University gently applied latex over all eight pairs of the
caterpillars' abdominal spiracles and then uncovered each pair
systematically while pinching the larva. The whistles definitely
came from the eighth pair, generating trains of whistles lasting
up to four seconds each, and spanning frequencies that ranged
from those audible to birds and humans up to ultrasound.

Silk-moth caterpillars make clicks to warn predators that they
would make nasty meals, so why do walnut sphinx caterpillars
whistle? To find out, Yack and Bura teamed up with researchers at
Queen's University in Kingston, Canada, who studied captive
yellow warblers (Dendroica petechia), a bird that is
known to frequently eat caterpillars and lives where the walnut
sphinx caterpillar does.

The scientists put walnut sphinx caterpillars on twigs in cages
with yellow warblers and patiently filmed the encounter.
Surprisingly, when the birds attacked, the caterpillars whistled
and the bird typically flinched and hopped or flew away. In tests
with three warblers and two attacks each, the caterpillars got
away completely unscathed. [ Video
of the whistling caterpillar ]

"These birds are clearly startled by the unexpected sounds coming
out of this caterpillar," Yack told LiveScience. "They dove for
cover."

The sounds are probably not advertising that the
walnut sphinx caterpillars are distasteful. The birds simply
appear to be startled, "because these sounds are unexpected,"
Yack said.

The scientists detailed their findings online Dec. 10 in the
Journal of Experimental Biology.